1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a protective element.
2. Description of the Related Art
A protective element is connected to the input and output terminals of a semiconductor circuit (i.e., an internal circuit) of a semiconductor device, to protect the internal circuit from electrostatic energy penetrating from the outside by discharging the energy to a ground or a power source line.
FIG. 7 is a circuit diagram showing a standard protective diode 1 serving as a protective element for an internal circuit 2 of a semiconductor device. The protective diode 1 is connected between an input terminal 3 and a ground terminal 4 of the internal circuit 2, to produce an inverse bias with respect to an input voltage VDD. A pn junction of the protective diode 1 undergoes a breakdown phenomenon responsive to positive static electricity applied to the input terminal 3, to thereby discharge the static electricity to the ground terminal 4.
FIGS. 8(a) and 8(b) are sectional views showing a part of the semiconductor device. The left-hand side of each figure shows a vertical bipolar transistor 5 serving as the internal circuit 2 of FIG. 7, and the right-hand side thereof shows the protective element 1 formed on a p-type silicon substrate 6 on which the vertical bipolar transistor 5 is also formed.
To simplify the fabrication, a silicon layer 8a of the protective element 1 and a silicon layer 8 serving as a collector of the vertical bipolar transistor 5 are both formed on the silicon substrate 6, to the same depth and through the same process. The protective diode 1 has a p.sup.+ -type diffusion layer 10a and an n.sup.+ -type diffusion layer 11a, and the bipolar transistor 5 has a p.sup.+ -type base diffusion layer 10 and an n.sup.+ -type emitter diffusion layer 11, in which the depth of the diffusion layer 10a is the same as that of the diffusion layer 10, and the depth of the diffusion layer 11a is the same as that of the diffusion layer 11.
To increase the withstand voltage (surge voltage) of the protective element 1, a design size of the protective element 1 to be designed is selected so that the size thereof is larger than that of the bipolar transistor 5, whereby a large amount of current can flow therethrough.
An n.sup.+ -type buried region 7 (7a) is formed between the silicon substrate 6 and the silicon layer 8 (8a), and an n.sup.+ -type diffused collector lead 9 (9a) is connected to the buried region 7 (7a). A p.sup.+ -type separation, or isolation, diffusion layer 12 electrically separates the protective diode 1 and transistor 5 from adjacent element regions.
The protective element 1 of FIG. 8(a) is a collector-emitter (C-E) short-circuited element in which the n-type silicon layer 8a is short-circuited to the n.sup.+ -type diffusion layer 11a through the diffused lead 9a and buried region 7a.
The protective element 1 of FIG. 8(b) is a collector-base (C-B) short-circuited element in which the n-type silicon layer 8a is short-circuited to the p.sup.+ -type diffusion layer 10a through the diffused lead a and buried region 7a.
In these two cases, the n.sup.+ -type diffusion layer 11a is connected to the input terminal 3 of the internal circuit 2 of FIG. 7.
When a sudden excess voltage (surge voltage) is applied to the C-E short-circuited protective element 1 of FIG. 8(a) in an inverse bias direction, a pn junction 13 between the n.sup.+ -type diffusion layer 11a and the p.sup.+ -type diffusion layer 10a breaks down due to an impurity concentration thereof, thereby absorbing the excess voltage and protecting the internal circuit therefrom. In this case, a current flows mainly through the side face of the n.sup.+ -type diffusion layer 11a.
When the surge voltage is applied to the C-B short-circuited protective element 1 of FIG. 8(b), a pn junction 13 between the n.sup.+ -type diffusion layer 11a and the p.sup.+ -type diffusion layer 10a breaks down. At this time, a current flows through the side face of the n.sup.+ -type diffusion layer 11a, and accordingly, a potential of the p.sup.+ -type diffusion layer 10a under the n.sup.+ -type diffusion layer 11a is increased. As a result, a pn junction between the p.sup.+ -type diffusion layer 10a and the silicon layer 8a is biased in a forward direction, and carriers move from the silicon layer 8a into the diffusion layer 10a, and thus the n-type silicon layer 8a, p.sup.+ -type diffusion layer 10a, and n.sup.+ -type diffusion layer 11a operate as an inverse vertical bipolar transistor.
In this way, in the C-B short-circuited protective element 1, the surge voltage is absorbed due not only to the current flowing through the side face 13b of the n.sup.+ -type diffusion layer 11a but also to the current flowing through the bottom 13a of the diffusion layer 11a. The current flowing through the bottom 13a is proportional to a current amplification factor hfc of the inverse vertical bipolar transistor. The C-B short-circuited protective element 1 of FIG. 8(b), therefore, reduces a concentration of current on the side face 13b of the diffusion layer 11a and increases the withstand voltage thereof more than the C-E short-circuited protective element.
The inventors of the this invention measured the inverse current amplification factor hfc of a sample device as IC/IB=0.6. In the C-B short-circuited protective element, the current flowing through the pn junction at the side face 13b of the diffusion layer 11a is dispersed, thereby to increase a withstand voltage at the side face 13b.
An emitter-base (E-B) short-circuited protective diode is known in which the p.sup.+ -type diffusion layer 10a is short-circuited to the n.sup.+ -type diffusion layer 11a. FIG. 9(a) shows an equivalent circuit of the E-B short-circuited protective element. This protective element uses a breakdown phenomenon occurring at a pn junction between the n-type silicon layer 8a and the p.sup.+ -type diffusion layer 10a.
According to the C-B short-circuited protective diode, the depths of the diffusion layers 10a and 11a are the same as those of the diffusion layers 10 and 11 of the vertical bipolar transistor 5 of the internal circuit, and a current density on the side face 13b of the n.sup.+ -type diffusion layer 11a is larger than that on the bottom 13a thereof, and thus the side face 13b is easily damaged. In addition, a surge current is concentrated at the side face 13b within a short time, because the discharge resistance thereof is low, and thus a sufficient withstand voltage is not obtained.
According to the E-B short-circuited protective element, the area of the pn junction between the p.sup.+ -type diffusion layer 10a and the n-type silicon layer 8a is made large, to thus obtain a high withstand voltage. The impurity concentration of the silicon layer 8a is low, however, and thus a breakdown voltage at the pn junction is large, and therefore, the E-B short-circuited protective element is not adequate for protecting an internal circuit having a low withstand voltage.
FIG. 9(b) shows a result of measurements made by the inventors for an applied voltage (V) and a corresponding resistance value (Q) for each of the C-B, C-E, and E-B short-circuited protective element. In the figure, the withstand voltage of the E-B type is 600 V, the C-B type 470 V, and the C-E type 130 V.
The protective element is required not only to have a high withstand voltage but also to disperse a current or reduce a current density by increasing a current flowing through the protective element. Therefore, preferably, the resistance of the protective element against an applied voltage is made as small as possible. In consideration of the above, the inventors found that the most preferable protective element is the C-B short-circuited type.